amiga floppy disk reading experience

Chuck Guzis cclist at sydex.com
Fri Jun 11 16:12:03 CDT 2010


On 11 Jun 2010 at 12:11, Eric Smith wrote:

> Note that the only data separator worth a damn in the WDC app note are
> the two that use PLLs.  The most commonly used was the one in Figure
> 14 that uses a 74LS629 VCO and the phase comparator of the WD1691. 
> The circuit by MPI in Figure 13 that uses an MC4024 VCO looks OK but I
> haven't seen it used.  The rest are crap.
> 
> Figure 11 uses a one-shot, and won't work reliably even for single
> density.  It's comparable to the worthless data separator built into
> the 1771.  Figure 12 uses a state machine and works approximately the
> same way the Apple II read state machine works.  It's probably good
> enough for single density, but won't work reliably for double density.

There seems to have been a general move toward "good enough" in the 
early 80s.  The first version of the IBM 5150 floppy controller used 
an IBM hybrid-packaged design; the second version used conventional 
TTL and a 4024 VCO that was good enough, but not as good as the first 
version.  I think later controllers (as used in the 5170) used the 
all-digital WD9216 type of data seaparator.  With the all-in-one FDCs 
(WD37C65, N82077), the question of data separator design became a 
nonstarter.  Just plug it in with no adjustments needed.

I suppose "good enough" accurately describes the Apple II scheme; 
"gutless wonder" would be another term that comes to mind.  GCR with 
the only error conrol being a one-byte arithmetic checksum on each 
sector.  Were there ever any "turbo" mods to run an Apple II at more 
than 2MHz without breaking the floppy logic?

--Chuck






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